Field of the Invention
The present invention relates to a correction method for an image forming apparatus, for correcting distortion and uneven image density of an image during image formation of a two-dimensional image by the image forming apparatus, e.g., a digital copying machine, a multifunctional peripheral, or a laser printer.
Description of the Related Art
In electrophotographic image forming apparatus such as a laser printer and a copying machine, there has been generally known a configuration to form a latent image on a photosensitive member with use of a light scanning device configured to perform scanning with a laser beam. In the light scanning device of a laser scanning type, a laser beam collimated with use of a collimator lens is deflected by a rotary polygon mirror, and the deflected laser beam is formed into an image on a photosensitive member with use of an elongated fθ lens. Further, there is known multibeam scanning in which a laser light source having a plurality of light emitting points is included in one package so as to perform scanning with a plurality of laser beams simultaneously.
Meanwhile, in order to form a satisfactory image without uneven image density and banding (stripe pattern caused by the difference in image density), it is desired that distances between scanning lines of which positions to be scanned with a laser beam are adjacent to each other in a rotational direction of the photosensitive member be equal to each other. However, the distances between the scanning lines are varied due to a plurality of factors described below. The distances between the scanning lines on the photosensitive member are varied by, for example, a fluctuation in a surface speed of the photosensitive member, or a rotation speed fluctuation of a rotary polygon mirror. Further, the distances between the scanning lines are also varied by a variation in angle of mirror faces of the rotary polygon mirror with respect to a rotary shaft of the rotary polygon mirror and a variation in intervals between light emitting points arranged on a laser light source. To cope with uneven image density and banding caused by such factors, there has been proposed a technology of correcting banding by controlling an exposure amount of the light scanning device. For example, in Japanese Patent Application Laid-Open No. 2012-98622, there is described a configuration in which a beam position detection unit configured to detect a beam position in a sub-scanning direction is arranged in the vicinity of the photosensitive member, and the exposure amount of the light scanning device is adjusted based on scanning distance information obtained from a detected beam position, to thereby make banding less noticeable.
Similarly to Japanese Patent Application Laid-Open No. 2012-98622 described above, as a configuration to make banding less noticeable by controlling an exposure amount, there is given a configuration to correct the positions of scanning lines by shifting image data in the sub-scanning direction in accordance with position information in the sub-scanning direction of each scanning line. In an electrophotographic image forming apparatus, banding is caused also by image positional deviation of from about 2 μm to about 5 μm. For example, in an image forming apparatus having a resolution of 1,200 dpi, the width of one pixel is 21.16 μm, and hence in order to correct the image positional deviation of from about 2 μm to about 5 μm, it is necessary to move an image gravity center with a resolution of 1/10 pixel or less. Meanwhile, when the image gravity center is moved by shifting (adding) image data, the movement amount of the image gravity center with respect to the image data to be added may be varied depending on the photosensitive member and the developing process conditions.
In FIG. 15A, exposure distributions (exposure areas) of two adjacent scanning lines overlap each other to form a composite light spot B in which light spots in the two scanning lines are added to each other. In FIG. 15B, the exposure amount of the first scanning line is decreased, and a pixel in the third scanning line is newly exposed to light in a small exposure amount. With this, a composite light spot A, in which light spots in the three scanning lines are combined, is formed, and it is understood that, as compared to the composite light spot B of FIG. 15A, the image gravity center of the composite light spot A is moved in the rightward direction of FIG. 15B. FIG. 15C and FIG. 15D are each a graph for showing a comparison of exposure widths and exposure positions obtained by slicing the composite light spots A and B with developing threshold values Th1 and Th2. As shown in FIG. 15C, when the composite light spots A and B are sliced with the developing threshold value Th1, the exposure positions of the composite light spots A and B are slightly shifted from each other, but the exposure widths thereof are substantially the same. Meanwhile, as shown in FIG. 15D, when the composite light spots A and B are sliced with the developing threshold value Th2, the exposure width of the composite light spot A becomes slightly thicker (wider) as compared to that of the composite light spot B, and the movement amount of the exposure position in the rightward direction of the composite light spot A also becomes larger. Further, as is understood from FIG. 16B, when a developing threshold value changes significantly at a time when banding correction is performed, the image density decreases in an image area A in which the image gravity center is moved as compared to that of an image area B in which the image gravity center is not moved, and a density change occurs. The details of FIG. 15A to FIG. 15D, and FIG. 16A to FIG. 16C will be described later.
As described above, in the case of performing processing of shifting image data by adding image data, there is a problem in that the movement amount also changes due to a change in developing threshold value, and hence banding correction cannot be performed satisfactorily. Such change in developing threshold value is liable to occur due to a change in image forming conditions, such as a charging amount for charging a photosensitive member, a developing voltage applied between a photosensitive member and a developing device, and an exposure light intensity.